Automatic analyzers, and in particular clinical chemistry analyzers, comprise an automatic pipetting unit with which pipetting operations are performed in a plurality of fixed positions. Even after thorough mechanical adjustment of the position of the pipetting needle during manufacture of the analyzer, the sum of the manufacturing tolerances of the various components of the analyzer and the deformations of the needle with time cause deviations of the position of the pipetting needle and make it difficult to have the pipetting needle properly aligned with the fixed pipetting positions it is expected to be positioned at by a transport device of the automatic pipetting unit. In order to have the pipetting needle properly aligned with the fixed pipetting positions, the operation of the analyzer has to include an initialization process which is carried out at each start of operation of the analyzer and which is suitable for positioning the pipetting needle at a reference, initial or home position, which in a Cartesian system is designated by the coordinates X0, Y0 and Z0 of the tip of the pipetting needle, and which is also called the zero position of the pipetting needle. Once the latter reference position is determined, the transport system of the pipetting needle should be able to position the needle accurately at each pipetting position.
In particular in compact analyzers, where the pipetting needle has to be introduced in vessels which have a relatively small cross-section, and where the distance between pipetting positions is relatively large, it is highly desirable to have a reliable initialization process of the above mentioned kind.
The task of providing such a reliable initialization process is particularly difficult when the transport device moves the pipetting needle only along a straight line, e.g. in X-direction only, and all pipetting positions are located in that linear path of the motion of the pipetting needle. A reliable initialization process is even more difficult to achieve when the portion of the pipetting needle which is introduced into a vessel for effecting a pipetting operation is moved along a circular path within the vessel for mixing liquids introduced in that vessel. In the latter case, a very accurate alignment of the pipetting needle and the vessel is required.
Known initialization methods require relatively expensive means. It is therefore desirable to have an initialization method which is reliable, even under the above mentioned circumstances, and which can be achieved at low cost.